Material recovery and separation from leftover printed circuit boards
Overview
The Waste Electrical and Electronic Equipment (WEEE) Directive (European Commission, 2003) mandates that member states do more than just dump waste electrical and electronic equipment in landfills; instead, they must repurpose, recycle, and recover it. According to Williams (2005), WEEE is made up of a broad range of devices, such as control instruments, TVs, computers, mobile phones, electrical tools, and medical equipment. The WEEE Directive mandates the ecologically sustainable processing of printed circuit boards as part of this waste management strategy. Printed circuit boards are found in many electrical appliances, including computers and TVs, as well as other appliances like washing machines, which are increasingly adopting printed circuit boards for functions like pre-programming and timers. Printed circuit boards provide unique recycling challenges due to their diverse composition of metals, glass fiber, and organic materials. According to Goosey and Kellner (2002), just 15% of the 50,000 tons of discarded printed circuit boards generated in the UK in 2002 were recycled. According to Fisk et al. (2003), waste printed circuit boards are currently either sent to landfills, where hazardous compounds may seep into the water supply, or burned, which may result in the formation of toxic brominated compounds from the brominated flame retardants contained in the circuit boards.
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Polymer films like polyimides, or less commonly polyethylene terephthalate or polyethylene naphthalate, or glass fiber composites bound with a thermoset resin, make up a large number of printed circuit boards. Difunctional epoxy resins like bisphenol A, multifunctional epoxy resins like epoxy novolacs based on phenol and creosol, BT epoxy blends, cyanate esters, and polyimides are examples of common resins. In addition to a resin, a hardener is required to form the cross-linking necessary to produce a thermoset plastic. Dicyanodiamide is the most often used hardener, although other options include 4,4′-diaminodiphenyl sulfone and 4,4′-diaminodiphenyl methane.
The selection of production materials for printed circuit boards is contingent upon the intended use. For instance, basic two-sided circuit boards can be adequately constructed using difunctional epoxy resins; however, thick multi-layered boards necessitate the use of more complex multifunctional epoxy resins or cyanate esters (Jawitz, 1997). However, televisions and home electronics primarily use printed circuit boards made of cellulose paper reinforced phenolic resin (FR-2), though high-end equipment increasingly contains FR-4 boards. The most common type of printed circuit board used in computers and communication equipment is made from glass fiber reinforced epoxy resin (also known as FR-4 commercially). Printed circuit boards are made of glass fiber, paper, and resin in addition to large amounts of metals, the most important of which is copper, which is utilized to create the electrical circuits on the printed circuit boards. According to Goosey and Kellner (2002), other metals found in printed circuit board trash include iron, nickel, silver, gold, and palladium from the solder used to affix the electrical components to the boards.
Pyrolysis is one potential technique for recycling printed circuit boards and obtaining the organic and non-organic portion. Pyrolysis is a thermal recycling process that has been extensively studied for its potential to recycle synthetic polymers, particularly those blended with glass fibers (Cunliffe et al., 2003), as well as synthetic polymers (Bhaskar et al., 2004, Brebu et al., 2005, Kaminsky et al., 2004, Hall and Williams, 2006). Gases, oils, and chars are produced when polymers undergo pyrolysis; these products can be used as fuels or chemical feedstocks. In addition, the solder used to join the electrical components to the printed circuit boards will melt during the pyrolysis process if the temperature is high enough. The organic material and the printed circuit boards’ organic percentage should be separated from the metal components with the help of solder removal and recovery.
Despite the fact that a substantial amount of research has been published on the pyrolysis of printed circuit board waste, the majority of these studies have used very small batch reactors or analytical pyrolysis techniques (Barontini and Cozzani, 2006, Barontini et al., 2005, Blazso et al., 2002, Bradna and Zima, 1991, Luda et al., 2005, Williamson et al., 1980). Furthermore, a significant portion of research on the pyrolysis of printed circuit boards has focused on understanding the makeup of the organic products—in particular, the brominated organics. Using a laboratory-scale batch reactor, we pyrolyzed a variety of different printed circuit board wastes in this study. In addition to characterizing the organic pyrolysis products, we also qualitatively identified the metals that may be retrieved from the waste boards. We have also discussed how simple it is to separate the glass fiber and metal components from the pyrolysis char as opposed to how challenging it is to remove metal components straight from printed circuit boards.
An additional study component looked at the chemicals that include phosphorus and bromine in the pyrolysis oil product. Flame retardants are a big worry when it comes to pyrolysis recycling printed circuit boards. To decrease the flammability of circuit boards and their components, brominated and/or phosphated additives—which are frequently toxic—are applied (Tohka and Zevenhoven, 2002). While FR-2 is fire retarded with penta-bromodiphenyl ether (penta-BDE) if the boards are made in Asia, FR-4 is fire retarded using tetrabromobisphenol A (TBBPA). However, the use of penta-BDE is declining (Lassen and Lokke, 1999). Manufacturers are employing alternative phosphate-based flame retardants, such triphenyl phosphate and tricresyl phosphate, more frequently as a result of the harmful nature of many brominated flame retardants (Lassen and Lokke, 1999). The pyrolysis of brominated circuit boards has been extensively studied (Balabanovich et al., 2005; Barontini and Cozzani, 2006; Barontini et al., 2005; Blazso et al., 2002; Chien et al., 2000; Luda et al., 2002). However, the authors were unable to locate much research on the pyrolysis of printed circuit boards that contained phosphate. We have examined the existence of phosphated and brominated chemicals in the pyrolysis oil in this work.